The present invention relates to a narrow gap welding method by which V-shaped or single-bevel grooved multi-layer build-up welding is carried out in MAG or MIG welding, and an apparatus for welding the same.
As civil construction machines such as a hydraulic shovels, etc., have been made larger and larger, steel plates that constitute structures such as arms, booms, buckets, etc., of the civil construction machines have become thicker and thicker, wherein a great deal of time has been required for welding work.
As one of the methods for shortening the welding time, there is a method for decreasing the total amount of weld by making V-shaped or single-beveled welding grooves into narrow gaps. However, in the case of making them into narrow gaps, an important factor becomes the prevention of welding defects from occurring particularly in the first layer.
For example, as shown in Japanese Laid-open Patent Publication No. 108275 of 1996, if members to be welded are tubular, the end faces thereof are very accurately machined to form welding grooves, and automatic welding may be performed. But, as in the structure of a civil construction machine, if welding grooves are formed, by machining, in large-sized members having complicated outer profiles consisting of straight lines and curved lines, the working costs thereof will become very expensive, wherein the costs of welding cannot be absorbed, resulting in an increase in the total production costs.
In the welding of welding grooves, for example, where the welding current is small in the first layer, the amount of heat input into members to be welded is low. Therefore, as shown in FIG. 7, a space 4a that cannot be buried by the first layer welding bead 3 occurs on the bottom of a welding groove 2 formed on a pair of welding matrices 1 as a welding defect.
If the welding current is increased, as shown in FIG. 8, while a sufficient amount of heat input can be obtained with respect to the welding matrices 1, the amount of a filler metal that melts is increased. Therefore, the longitudinal and latitudinal ratio (ratio of height to width of a welding bead) of a welding bead 3 is accordingly increased, wherein a welding defect such as a hot crack 4b occurs in the welding bead 3.
In order to sufficiently secure penetration with respect to a welding matrix and prevent a hot crack from occurring in welding beads, it is necessary that the welding current is increased, and at the same time the height of welding beads formed at the welding portions is suppressed to be low by making the welding rate faster.
Also, as shown in FIG. 9, as a welding wire 6 approaches one side wall shifting from the center position of the groove 2, an arc 7 flows to the side wall where the welding wire approaches, wherein after the welding is finished, a larger undercut 4c occurs as shown in FIG. 10. Also, since the center of the arc is not oriented toward the bottom portion of the groove 2 (the directivity is low), there may be a case where penetration at the bottom portion of the groove 2 becomes insufficient.
The directivity of an arc in welding can be obtained by a pinch force operating on the arc. As one of the pinch forces operating on the arc, there is an electromagnetic pinch force that is generated by a welding current. The intensity of the electromagnetic pinch force is proportionate to a square of the welding current.
Therefore, in the first layer welding in the structure of a civil construction machine, etc., it is difficult to employ a welding method such as semi-automatic welding in which a serpentine run of a welding wire or a deviate of the target position due to hand shake is unavoidable. The welding has manually been carried out by a skilled welding worker, wherein it is difficult to decrease welding costs.
In view of the abovementioned situations, it is therefore an object of the invention to provide a narrow gap welding method and a welding apparatus therefor, that enables the application of automatic or semi-automatic welding in a first layer in narrow groove welding, by improving the directivity of an arc generated in welding, and increasing allowance with respect to a serpentine run of a welding wire or a deviate of the target position due to hand shake.
In order to achieve the object, a first aspect of the invention is a method for welding narrow V-shaped or single-beveled grooves by MAG or MIG welding, comprising the steps of welding in a first layer by applying a welding current by which a globule transfer pattern is made into a spray pattern by a waveform in which a direct current and a pulse current are caused to overlap between a welding matrix and a filler metal, and thereafter welding the second layer by applying a direct current.
A second aspect of the invention is featured in that the abovementioned direct current is made into 200 A through 300 A while the peak value of the pulse current overlapped on the direct current is made into 400 A through 500 A.
A third aspect of the invention is featured in that the frequency of the abovementioned pulse current is made into 50 Hz or more.
A fourth aspect of the invention is an apparatus for welding a V-shaped or a single-beveled groove by MAG or MIG welding, which comprises a welding power source that applies welding power to a welding portion composed of a welding matrix and a filler metal, a current controller that compares a predetermined welding current with a welding current supplied to the welding portion and transmits a current controlling signal, a voltage controller that compares a predetermined welding voltage with a welding voltage supplied to the welding portion and transmits a voltage controlling signal, a pulse waveform controller that transmits a pulse waveform signal based on a predetermined peak voltage, base voltage and frequency, and an output controller that controls an output of the abovementioned welding power source based on outputs of the abovementioned current controller, voltage controller and pulse waveform controller, and applies required welding power to the abovementioned welding portion.
A fifth aspect of the invention is an apparatus for welding a V-shaped or a single-beveled groove in a multi-layered build-up by MAG or MIG welding, which comprises a mechanism for moving a welding torch, a controller that controls the abovementioned mechanism so that the abovementioned welding torch is moved relative to a welding matrix along a welding line of the welding matrix, a welding power source, in which a plurality of welding current waveforms are set, that applies welding power between the abovementioned welding torch and welding matrix, a pulse waveform controller that generates welding current waveforms applied between a welding matrix and a filler metal in the first layer of the welding matrix, and an output controller that generates a direct current waveform when welding the second and subsequent layers of the welding matrix, and simultaneously controls an output waveform of the abovementioned welding power source, placing priority on the welding current waveform when a welding current waveform is inputted from the abovementioned pulse waveform controller.